Put into plainer English, the Bureau warns that union membership is more common among jobs that pay more for reasons that may be entirely unrelated to unionization itself. Big-city jobs often pay more than their rural counterparts, big businesses often pay more than small businesses, skilled employees usually get paid more than unskilled employees; unionization may be more common among certain types of jobs (big city, big business, high skills), but that doesn’t mean that unions necessarily cause employees to live in cities, work for big firms, or have useful skills. Add in the fact that union organizers have an incentive to seek to represent employees with higher incomes in the first place (since they can fund a better lifestyle for the union staff).